Abstract
The possibility to evaluate the molecular diffusivity in polymer thin films used for packaging and device encapsulation directly in-situ would represent a paradigm changer in the assesment of barrier properties and of air quality. Indeed, employing the packaging itself as a smart sensor could lead to waste reduction and mitigate food poisoning effects. In this work, we demonstrate a new technique that exploits simple UV-Vis reflectance spectroscopy to identify the kinetic of diffusion of small molecules in the vapor phase through polymer thin films and polymer multilayered structures. The new method allows then to assess the presence of the analyte in air and its diffusion coefficient in agreement with gravimetric data reported in literature.
Highlights
Diffusion coefficient of vapor molecules through polymeric films is usually determined by methods such as gravimetry[1] and pressure decay,[2] or by optical techniques like microscopy,[3] infrared spectroscopy,[4] and refractive index variations.[5]
Polymer distributed Bragg reflectors (DBRs) made of thin films have shown large pollutants absorption that result in an optical response with kinetics depending on the pollutant themselves (Fig. 1) .[10,11,12,13,14,15,16,17,18,19,20]
To demonstrate that this simple method can be applied to unstructured polymer films, Fig. 2 reports the optical response for a DBR made of 10 bilayers of PS and cellulose acetate (CA) with total thickness of about 4 μm and compare it with the response
Summary
Diffusion coefficient of vapor molecules through polymeric films is usually determined by methods such as gravimetry[1] and pressure decay,[2] or by optical techniques like microscopy,[3] infrared spectroscopy,[4] and refractive index variations.[5]. The background of the spectrum of these structures is instead dominated by an interference pattern, that is typical of transparent thin film with small surface roughness The properties of these spectral features are subjected to the modifications of the lattice components refractive index and thicknesses.[31] perturbing these parameters affects the entire spectrum, and the spectral variations can be related to stimuli such as pressure variations,[32] chemical analytes,[14,15,33] and pH.[34] The red line of Fig. 1c displays the typical response of a polymer DBR spectrum after intercalation of an analyte. Notice that the nice similarity between the curves allow to perform the measurements in any part of the UV-Vis spectrum making the method suitable for non-transparent materials
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